The invention relates generally to subterranean gas wells. More particularly, the invention relates to systems and methods for producing a single formation from a gas well using multiple production tubing strings.
Geological formations that yield gas also produce liquids that accumulate at the bottom of the wellbore. In general, the liquids comprise hydrocarbon condensate (e.g., relatively light gravity oil) and interstitial water from the reservoir. The liquids accumulate in the wellbore in two ways—as single phase liquids that migrate into the wellbore from the surrounding reservoir, and as condensing liquids that fall back into the wellbore during production of the gas. The condensing liquids actually enter the wellbore as vapors; however, as they travel up the wellbore, their temperatures drop below the respective dew points and they change phase into liquid condensate.
In some hydrocarbon producing wells that produce both gas and liquid, the formation gas pressure and volumetric flow rate are sufficient to lift the liquids to the surface. In such wells, accumulation of liquids in the wellbore generally does not inhibit gas production. However, in wells where the gas does not provide sufficient transport energy to lift liquids out of the well (i.e., the formation gas pressure and volumetric flow rate are not sufficient to lift liquids to the surface), the liquids accumulate in the wellbore.
For example, referring now to FIG. 1, a conventional system 10 for producing hydrocarbon gas from a well 20 is shown. Well 20 includes a wellbore 26 that extends through a subterranean formation 30 along a longitudinal axis 17. System 10 generally includes a wellhead 13 at the upper end of the wellbore 26, a production tree 12 mounted to wellhead 13, a primary conductor 21 extending from wellhead 13 into wellbore 26, a casing string (“casing”) 22 coupled to wellhead 13 and extending concentrically through primary conductor 21 into wellbore 26, and a tubing string 40 coupled to tree 12 and extending through casing 22 into wellbore 26. An annulus 27 is formed between string 40 and casing 22. Tree 12 includes a plurality of valves 11 configured to regulate and control the flow of fluids into and out of wellbore 26 during production operations.
During operation, formation fluids (e.g., gas, oil, condensate, water, etc.) flow into the wellbore 26 from a production zone 32 of formation 30 via perforations 24 in casing 22. Thereafter, the produced fluids flow to the surface 15 through annulus 27. In most cases, the production zone 32 initially produces gas to the surface 15 through annulus 27 with sufficient pressure and volumetric flow rate to lift liquids that enter wellbore 26 from zone 32 through perforations 24. However, over time, the formation pressure and volumetric flow rate of the gas decreases until it is no longer capable of lifting the liquids that enter wellbore 26 to the surface 15. At some point, the gas velocity drops below the “critical velocity”, which is the minimum velocity required to carry a droplet of water to the surface. As time progresses, droplets of liquids accumulate in the bottom of the wellbore 26, thereby forming a column of liquid. This column of accumulated liquids imposes a back-pressure on the production zone 32 that begins to restrict the flow of gas into wellbore 26, thereby detrimentally affecting the production capacity of the well 20. Consequently, once the liquids are no longer lifted to the surface by the produced gas, the well eventually becomes “loaded” as the liquid hydrostatic head imposes a pressure on the production zone sufficient to restrict and/or prevent the flow of gas from the production zone, at which point the well is “killed” or “shuts itself in.”